Why use 50 ohms for RF cable assemblies? Jun 21, 2022

    Most of the cable components seen in the market are basically 50 ohms as the transmission line impedance, so why not 60 ohms or 70 ohms? There is a lot of little knowledge here. The following  will tell you why the RF cable assembly uses 50 ohms instead of 60.70.80 ohms.

    Look at the point of loss. There is a skin effect in high frequency high speed lines. The industry has proven that 50Ω has the smallest loss for the skin effect. Usually the skin effect loss L of the cable (in decibels) is proportional to the total skin effect resistance R (unit length) divided by the characteristic impedance Z0.

    Total Skin Effect Resistor - [R is the sum of the shield and intermediate conductor resistance. The skin effect resistance of the shield is inversely proportional to its diameter D2 at high frequencies. The skin effect resistance of the inner conductor of the RF coaxial cable is inversely proportional to his diameter D1 at high frequencies. The total series resistance R is therefore proportional to (1 / D 2 + 1 / D1). Taking these factors into account, given the dielectric constant of D2 and the corresponding isolation material, the ratio of D2 / D1 can be calculated with minimal skin effect loss.

    Assuming that the dielectric constant of solid polyethylene is 2.25 and the skin effect loss is minimal, D2 / D1 = 3.5911 gives a characteristic impedance of 50 ohms.

    The impedance is mainly determined by three factors: line width, copper thickness and dielectric thickness. The three parameters are 5.5 mil, 1.4 mil, and 3.5 mil. These parameters are relatively easy to manufacture for production, the impedance is small, the smaller the thickness of the medium, the smaller the thickness of the medium will exceed the process capability of the production equipment. 5-6mil is now available to general PCB manufacturers. Linewidth is a bit of a design engineer who has to complain about high-density, high-speed PCBs. So it is not surprising that 50Ω for RF cable assemblies is the standard in the industry.

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